Flues or chimneys need to be heated to vent?

I think it goes back to old time thinking when people would light those huge fireplaces, first with some paper to 'heat up the chimney'.
But I've never heard anything about heating up flues for them to work. If anything, clean an oil fired piece of equipment in the dead of winter on a service call that's been sitting for hours, and as your brushing the thimble, soot is being sucked up the chimney.

that flue “Should” always be warmer and exhausting building air.

I think this is more of a fireplace flue question.

I remember the ball of newspaper trick in the home where I grew up. It worked for the 18 years I lived there.

what about undersized? why does the code say in certain combinations of common venting you have to upsize the vent after it leaves the appliance?

Hot air doesn't rise, it gets displaced by cold air.

it gets displaced as it rises, yes

Semantics: Isn't cold just the absence of heat, isn't dark just the absence of light, isn't evil just the absence of good.

I think that the hot air rises and is replaced by cold air

You say that cold air displaces the hot air.

I think Sandra Bullock is hot. You might think that Julia Roberst is a Pretty Women, I had a friend that was a Marlyn Monroe fan.  None of them have anything on @Erin Holohan Haskell     That might be off topic. Oops Sorry!

The net outcome is that if the air in the vertical vent pipe is lighter than the air surrounding the vertical vent,  then you should have no problem getting the smoke out.  

Where you run into problems is when other factors like a building with an extreme negative pressure, or a tree grown above the top of the vent exit, or there are hills or buildings that cause the wind from a certain direction to act as an air scoop make downdraft problems, you need to address those problems in some fashion or another.  

I think that @Bob Harper should comment herein.

I believe the Venturi effect or Bernoulli’s theorem is part of the equation, too.

Jim and I have gone rounds over this one. Start with the campfire/ candle scenario: it is surrounded by relatively cool, homogenous air. You introduce a flame, which reduces the density of the flue gases relative to the surrounding air, this the flue gases rise as they are being displaced by the colder, denser air. Out in the open, it has to go somewhere so it fights and forces itself upwards against gravity and the falling cool air. This, combined with the Coriolis Effect, cause the characteristic helical flame/ smoke pattern. Being exposed to the cooler air, this column quickly cools to the surrounding equilibrium instead of rising to space so it diffuses out.

Now, slide a conduit over that flame leaving both ends open. The conduit is full of cold, dense air that cannot be pushed aside as it was unrestricted. Therefore, it takes the path of least resistance. If it has sufficient energy, defined as the difference in air density vs. flue gas density, the flue gases may be able to force that column of cold dense air up and out similar to a push-pop or a piston. However, if there is sufficient relief and a lower pressure gradient in the CAZ, it will backdraft. Most CAT I appliances do this at startup. ANSI allows this nonsense to go on for 10 minutes. That's because when it was written houses still leaked like a sieve. It didn't matter. Now we live in Tupperware.

If the walls of the conduit are cold relative to the flue gas column, they will naturally absorb BTU's until they both are at equilibrium all the way to the outer wall of the flue liner. If this liner is terra cotta properly installed, you have about 1" of clay to heat. If it's mortared in as most masons from the old country did it, the heat continues on all the way through the mass of masonry until it either reaches a capillary break or the outer wall. This is why skinny chimneys heat up more quickly than huge, high mass chimneys. It also explains why high mass chimneys that are kept hot all season rarely suffer chimney fires while the skinny ones burned for short periods do gunk up with creosote (wood) or acidic condensate with gas or oil, and rot. A hot flue is a happy flue.

If BTU's are being robbed to heat the walls of the flue liner, less heat is available to reduce the flue gas density, thus it hinders creating draft pressure. Thin-walled, low mass metallic liners heat up very quickly, thus leaving those BTU's to generate the requisite draft pressure differential to get mass flow, popularly erroneously referred to as "draft". Draft is pressure- not flow. So, how does this factor into Jim's question? For me, it seemed quite obvious he was dead wrong from all the above: you MUST heat the flue liner walls and cold flue air at standby before mass flow upwards can begin. Binary thinking. As I got into things, I discovered impossible situations.

For instance, I had a gas direct vent where the concentric fresh air intake had been 100% blocked for two years yet the appliance periodically ran fine. As crazy as it sounds, this unit forced itself to pull sufficient air down the exhaust flue passing the flue gases going the other way. It set up a two way stream. This is what happens in larger flues or the outdoors. A small, tight column of helical flue gases will fight its way up past the falling cold air. If firing is sustained, more heat is transfered to the flue liner thus leaving more heat for draft pressure until virtually all the air in the stack becomes flue gases.

Except, on cold exterior stacks, you get a permanent downward flow of air hugging the flue liner walls. This happens in hot air balloons and exterior walls in heated buildings. This happens in large smoke stacks. Despite years of seeing non-stop gazillion BTU's of input, the walls only get just so hot. That boundary layer creates an airwash of cooling air that simply cannot be fully overcome. The high thermal mass, usually 3 wythes thick at the upper reaches, does store heat but only enough to stabilize the draft during high winds, cold weather, rain, etc. Rain is another problem altogether cooling the walls and flue gases unless there is a rain cap.

On small venting systems, such as residential, I think the phenomenon of a cold backdraft from a large stack can be significant. However, as you approach the 10" or so/ 1 million BTU club, the two-way phenomenon is definitely real. To be sure, I see MOST residential flues cold backdraft, which is partially solved by a properly sized low mass metallic liner. The other part of the equation is CAZ depressurization, which is a whole another discussion. On most 4 story apartment buildings and similar commercial stacks, I find a positive flow at standby. This, I think is partially due to how much heat is being lost up the stack from these old inefficient boilers and partially by the length of firing cycles with extended wet times in between. It's a weird juxtaposition between dewpoint and minimum draft pressure for positive flow versus stack losses and firing cycles.

I have seen more flow reversals and backdrafting in residential applications with oversized flues, but again, those also typically have negative pressure regimes and cold exterior chimneys serving orphaned water heaters. The ~10 MBH stack losses on a 40 MBH DHW heater into a cold, wet 2 story masonry chimney with a 3" vent connector will backdraft for most, if not all of its firing cycle. A ranch house with a nominal 8"x8" flue also struggles to develop sufficient lift. Here, the larger flue, ONCE HEATED, generates more lift than a smaller volume flue. Think of flues as hot air balloons. The bigger the greater lift- once the whole balloon is full of hot air. BTW, air inside hot air balloons slides down the walls of the balloon and out as ambient air is entrained up into the bag, similar to our discussion.

Now, can you take a commercial building with a 16" square ID TC flue that once served a couple million BTU's firing almost constantly then expect it to behave predictably when you replace those old inefficient boilers with much higher efficiencies firing at markedly reduced input rates? You might get it to work but you will still have to contend with firing cycles, negative CAZ pressures and condensation.

See attached DV handbook for fun.

Howzat Jim?

Why do flues in the summer on a water heater have a lower draft? Because I said So!

Are flues heat exchangers that are supposed to cool the flue gases down? Maybe

Why were the flue venting tables produced by a computer and not field testing? Laziness

If you put some oil in the bottom of a glass does it rise or get displaced when adding water? YES

Do Flue Liners increase flue temperatures avoided condensation? No, Flames increase flue temperatures. Anything after that will cool flue temperatures. DAH!

Why do the GAMA/AGA state they cannot be used in windy conditions? Nobody knows

What flue material keeps the flue gases the hottest and not absorb excess heat? Good Question!

When venting into a chimney, we want the flue temperature to stay hot until the exit. Agree!

I hope this makes the whole subject clear as mud!

The best natural flue designs are those which can help establish draft quickly, limiting the “wet time” of operation of the flue/appliance. On cold starts all appliances will condense. The trick is to use materials which will heat and dry quickly which will allow the appliance to establish proper draft conditions. That material is never masonry. Masonry never looses its appetite for a btu. Sizing is critical. Take a 4” breech of 12.5 sq in. Throw it into a 6” (28.26 sq in) or 8” (50 sq in) chimney and that nice concentrated pile of btu in the 4” hits that cold mass of stacked air and it just goes ”proof” as it diffuses into the larger space attempting to get the draft moving. Compounding that is inadequate CA.

The gas codes changed in ’92.You could cover up a lot of design deficiencies with a high stack temp on a 6o-65% appliance but with the high stack temp but at a minimum 78% all hell was breaking loose. We trained using that wet time concept. I think App E was added to 31 in about ‘00. It was stuffed into an appendix due to the cost of relining. The physics and testing were done at Brookhaven, but his was a cost issue, gas/oil.